That is factually incorrect. Not even the archduke can void (or worse break) a magic contract. A magic contract can only be altered or nullified if all people who signed it agree to the change. The most the archduke can do is order the people who signed it to change the terms of the contracts themselves, but he can't change the contract by himself.
The magic contract is not really made by the archduke, the contract is technically enforced directly by (part 5 spoiler) the Goddess of Light herself.